If you think about the way the fuel gauge works, it makes perfect sense. There is an object that floats at the top of the fuel that tells how full the tank is by its position in the tank. Well if the tank is full, then the floating object will be at the very top. The object would not move until the fuel not only goes down, but it has to go down beneath the bottom of the object. So, depending on the height of the object, you could have an inch or two of fuel consumed before it actually moves and therefore shows a change on the gauge. Hopefully, this make sense...
Btw, I call it an object because I don't know the real name for it.
